1. Field of the Invention
The present invention relates generally to aircraft actuation systems, and more particularly to a dual load path fail-safe actuation system.
2. Description of the Related Art
Modern aircraft have horizontal stabilizers located at the rear tail section of the fuselage or the forward section that are pivotally supported relative to the airplane fuselage to “trim” the aircraft during flight by selective adjustment by the operator or auto-pilot from an internal control unit. The stabilizer actuator is a variable length structural link connecting the horizontal stabilizer control surface to the fuselage structure and used to control the pitch (attitude) of the aircraft during takeoff, cruise and landing phases under different aerodynamic loading conditions. The stabilizer actuator is also used to recover the aircraft during severe aircraft stall situations. In this regard the stabilizer is traditionally pivotally connected to the rear section (or tail section) or forward section of the fuselage.
One common trimmable horizontal stabilizer actuator consists of a primary ball nut assembly connected with an actuating drive gimbal which is pivotally connected to one end of the horizontal stabilizer structure. The ball nut assembly includes a ball nut housing and a rotatable ball screw extending axially and usually vertically through the ball nut housing and a drive gimbal housing. The ball nut housing is connected to the drive gimbal housing by a trunnion segment. The ball screw, in turn, has one end remote from the actuating drive gimbal and is fixed from translation or axial movement by a connection to a second, support gimbal which is pivotally secured to the vertical stabilizer section or the tail section. As the ball screw is rotated, the drive gimbal will be moved in translation relative to it. Thus as the ball screw is rotated in one direction, the leading edge of the horizontal stabilizer is pivoted upward, whereas by rotating the ball screw in the other direction, the leading edge of the horizontal stabilizer is pivoted downward achieving the desired or commanded horizontal stabilizer angle. Rotation of the ball screw is routinely done by a motor (electric or hydraulic, depending on system architecture) and associated gearing which is connected to the second, fixed support gimbal and which is actuated by the operator or pilot by the internal control unit. The connection of the stabilizer actuator to the stabilizer is located within the vertical stabilizer or fuselage tail section and not directly in the air stream.
The horizontal stabilizer movement, as controlled by the operator or auto-pilot, is transmitted by the ball screw through the actuating drive gimbal by way of the primary ball nut assembly which defines a primary load path. The movement has a load with tensile and compressive components as well as a torque component due to the ball screw thread lead. Failures of the primary load path such as caused by the shearing off of the connecting trunnion segment, ball screw disconnect or by the loss of nut ball members from the ball nut assembly can result in the complete loss of control of the horizontal stabilizer. However, stabilizer actuators have frequently been provided with a secondary load path for alternate control of the stabilizer and structural integrity, as well as to meet the required level of safety (failure of single load path actuator has a catastrophic outcome on the aircraft). In such structures, the primary load path is normally controllably actuated by the operator and is thus under load while the secondary load path is normally unactuated and thus unloaded in standby mode. The secondary load path is maintained unloaded during intact primary load path by means of designed in gaps assuring that no load sharing will occur between primary load path components and secondary load path components when the primary load path is axially loaded. In the event of a primary load path failure, the secondary load path is automatically mobilized whereby the stabilizer actuator is jammed in position by means of locks (tie-rod lock or secondary inverted nut lock) and can no longer continue to be controllably actuated by the operator, pilot or auto-pilot to position of the stabilizer. The transition of control to the secondary load path can occur quite rapidly whereby failure of the primary load path is detected by the operator or pilot by means of the jammed actuator.
However, the engaged secondary load path and jammed actuator will have a large axial backlash which in the event of repeated load reversions could enter into a oscillatory mode that will cause rapid deterioration of the secondary load path structural integrity leading to a catastrophic failure condition. The present invention offers a method and solution for allowing sufficiently large gaps between the primary load path and the secondary load path components to prevent load sharing between the two paths during normal operating conditions (when primary load path intact). Yet, when the primary load path has failed and the secondary load path is engaged the present bidirectional locking mechanism will trigger minimizing the axial backlash of the secondary load path to allowable levels assuring the actuator in the secondary load path condition is unaffected by a flutter condition.